This invention is an improved nuclear fuel element incorporating a burnable poison. The poison would be contained in a thin layer of glass on the inside of the fuel rod cladding. This is a specific improvement in the invention disclosed and covered by U.S. Pat. No. 4,990,303, issued Feb. 5, 1991, to the same joint inventors and assigned to the same assignee.
The invention relates to nuclear fuel elements and, in particular, fuel elements with a burnable poison coating in the form of a thin layer of boron carbide. The boron carbide coating contains the boron-10 burnable poison isotope and is composed of particles sealed to the nuclear fuel element tube wall with an alkali metal borate glass deposited on the inside of the zirconium alloy cladding tube by sol-gel technology.
A nuclear fuel element of the type involved in the invention is part of a fuel assembly. Heretofore, typically fuel assembly designs have employed fixed lattice burnable poison rods to control early-in-life reactivity and power peaking. These rods have become a necessary design feature for the fuel management of first cores of light water reactors as well as in schemes to achieve extended burnups and reduced radial neutron leakage. Such rods displace fuel rods within the assembly lattice which increases the core average linear heat generation rate and local peaking factors. Alternate approaches have been proposed that place burnable poison material inside the fuel rods so that much less fuel material is displaced, for example, as boride coatings on the UO.sub.2 pellets. Such coatings, however, while adhering when first applied, tend to spall off under the stresses of the irradiation environment in the nuclear reactor core, in part because of difficulty in matching the thermal expansion behavior of the coating to that of the fission material or UO.sub.2 pellet. Attempts to incorporate boron compounds as mixtures within the UO.sub.2 pellets have not been successful because of volatilization of boron species during high temperature fabrication processes and redistribution of the boron under irradiation.
The invention involves an improved fuel element with a burnable poison coating which substantially overcomes problems of spalling and coating integrity because of the closely matched thermal expansion coefficients of the substrate and coating material and the action of fission sintering to enhance adhesion of the coating to the substrate.
In the above referenced patent (U.S. Pat. No. 4,990,303), a sol-gel derived glass containing boron-10 was disclosed. The glass could be applied to form a dense, adherent, abrasion-resistant coating on the inside zircaloy tubes at temperatures below 400.degree. C. Although the glass technically met all the requirements for a burnable poison, the application procedure was complicated and not desirable for large scale production. Specifically, the glass had to be applied in successive layers and each layer cured in order to achieve good adhesion. A one-step process for applying the boron-10 coating is more desirable.
For further background, see U.S. Pat. Nos. 3,925,151; 4,372,817; 4,560,575; 4,566,989; 4,582,676; 4,587,087; 4,587,088; 4,636,404; 4,824,634; 4,880,597; and 4,990,303.